1. Field of the Invention
The invention relates to the field of ferrous and nonferrous metal casting and in particular to the debonding and removal of sand cores from cast parts, and in some cases, the heat treating of the cast parts in conjunction with the removal of sand cores.
2. Brief Description of Related Art
In the casting of ferrous and nonferrous metals into parts, the foundries in the United States consumed 7.7 million tons of foundry sand in the year 1988 alone. The steel foundries and many of the gray iron foundries use high purity (over 98% by wt. SiO.sub.2) silica sand for casting molds. Many of the automotive foundries use a less pure (over 93% by wt. SiO.sub.2) silica sand. Most of this sand is used by the foundries for molding or core making. When making molds or cores, a binder material is added to the foundry sand to form the mold or the core. In general, the mold forms the outside surfaces of the casting, while the cores form the inside surfaces and paths. The cast part is formed by pouring the molten ferrous or nonferrous metal into the mold. When the part has internal openings or paths, the molten metal is poured into the volume between the mold and the core(s) usually surrounding some or most of the core. When the metal solidifies, the mold is opened and the part is removed. In most cases, the core remains in the interior regions its presence has formed and must be removed.
Removal of the cores is usually accomplished by impact and vibration devices, and/or by heating to destroy the binders and/or manually by breaking and prying out of the cores. The cores are generally broken into smaller pieces within the part and can be removed through various part openings. The degree of difficulty of doing this "sand core debonding" depends upon the geometry of the part being cast and the temperature of the metal melt.
In the case of casting parts of aluminum or aluminum alloys, it is particularly difficult to remove the sand core because of the lower casting temperature used. A lower interface temperature, usually results in less separation of the sand core from the aluminum part. The aluminum also is a softer material and more prone to damage if physical impact is used in the debonding and removal process. In addition, it is necessary to cool the aluminum part substantially before any attempt is made to debond and remove the sand core by any reasonable physical means, or the part will be damaged by even modest handling.
When heating methods are used to remove sand cores by thermal destruction of the binder systems, heating cycles are typically long, 4 to 10 hours, and the removal of the core is frequently incomplete. Pieces of sand core remain where the heating process did not effectively thermally decompose all parts of the sand core. Additionally, sand core material removed from the castings must be disposed of or reclaimed. Disposal has become increasingly expensive because the binder residue is usually classified as a hazardous and/or toxic waste which must be handled accordingly. Reclamation of the foundry sand through physical and thermal processing steps is receiving increasing attention, but also involves a significant cost.
U.S. Pat. No. 5,423,370, which is incorporated herein by reference, describes the invention of a fluid bed furnace for the removal of sand cores from castings, employing a thermal process based on the use of fluidized sand of the same type as used to make the sand core. This same patent describes the use of the fluid bed furnace for the heat treating of the aluminum castings. This fluidized sand thermal process eliminates the major disadvantages associated with conventional sand core debonding processes.
However, the invention described in U.S. Pat. No. 5,423,370, depicts practicing the process using a batch fluid bed process; i.e., the parts being processed are placed in or on a basket or containing fixture and are then submerged in the fluidized solids at a suitable temperature for a suitable period of time to pyrolyze and/or otherwise thermally decompose the sand core binder thereby releasing this sand to flow freely into the fluidized bed and ultimately be recovered and reused.
For applications involving high volume processing of parts, the casting machines are typically designed to form the casting by a relatively short cycle repetitive casting operation.
The use of a batch fluid bed furnace or furnaces to perform the sand core debonding and/or simultaneous or subsequent heat treating operations exhibits the following disadvantages:
a) After the parts are cast, they are introduced into fixtures or baskets until these holding devices are filled to their capacity, where upon the fixtures or baskets containing the parts are submerged in the fluid bed furnace for the time required to accomplish the processing objectives.
This requires the first parts entering the fixture or basket to wait until the loading of the basket or fixture is completed thereby losing heat during this waiting period. The average temperature of the parts in the loaded fixture is considerably lower than their temperature when they leave the casting machine. This represents energy inefficiency with respect to a following thermal process for sand core debonding and heat treating.
b) In typical applications of high volume processing of castings, the casting machines are delivering parts to the process at a uniform cycle time. The requirement to receive a load of parts, to open the fluid bed furnace cover and load the parts, then close the furnace cover, adds time to the processing cycle time; thereby increasing the cost of the process.
In addition, the uniform conveying of the parts through the casting process is interrupted by the batch nature of the fluid bed furnaces and would be more effectively served by a continuous or semi-continuous flow of product through a continuous or semi continuous fluid bed furnace for sand core debonding and heat treating.
This invention involves the use of a continuous or semi-continuous fluid bed furnace for sand core debonding of ferrous and nonferrous castings with or without subsequent heat treatment. This invention eliminates the disadvantages of the older non-fluidized bed processes as well as those of the batch fluid bed furnace, achieving a more effecting processing system with respect to operating cost as well as processed part quality.